Method of digesting cellulosic material



Aug. 31, 1937. A D MER 2,091,513

METHOD OF DIGESTING CE LLULOSIC MATERIAL Filed Feb. 14, 1955 S E i b 8 Snow,

@atented Aug. 31, 1937 UNITED sTATEs PATENT oFFicE METHOD OF DIGESTING CELLULQSIC MATE RIAL

2 Claims.

' material.

In the older methods of digesting fibrous material, for the production of pulp by the quick cook method, the digester was filled with chips and acid liquor was then admitted. The mass was brought up to reaction temperatures by admitting direct steam. In this older practice the steam was admitted quite slowly for it was generally thought that this must be done to permit the acid to penetrate the interior of the chips. Thus it was necessary to so control the admission of steam that a temperature of from 110 C. to 120 C. was reached only after a period of from two to five hours.

It has recently been found, as disclosed particularly in United States Patent to Dunbar 1,359,384, that improved results are secured when the chips are saturated with hot acid, under high hydrostatic pressure, prior to the cooking period.

The present process is of the latter type but includes certain novel steps in the treatment whereby the process is facilitated and a saving in time and other advantages secured.

It is now known that a greater yield and higher grade of pulp may be obtained by submitting the chips to a thorough saturation with hot acid under high pressure and so controlling the conditions during the saturating period to insure uniform temperature throughout the mass. This may efiectively be done by recirculating hot liquor below reaction temperature, and under a high hydrostatic pressure, directly to and from the digester. In one method proposed heretofore relatively concentrated acid liquor, at a temperature of C. to C. or more was stored in a pressure vessel, called the accumulator, and was pumped directly from this to a digester which previously had been filled with chips. When the digester was filled pumping was continued until a high hydrostatic pressure of the order of six atmospheres more or less was established. This mass of high pressure acid liquor was then recirculated to and from the digester to equalize the temperature throughout the mass of chips and preferably heat units were added to raise the temperature to the optimum saturating value, ire. of the order of C. to C. After a saturating period of two hours more or less, under these conditions, the conditions within the digester were adjusted to initiate and complete the cook.

Such a system has operated very successfully, showing striking economies. In the use of this through the individual chips is assured and a maximum yield of high quality pulp is achieved. I

type of unit, however, a large accumulator was required inasmuch as all the liquor pumped to the digester was drawn from it. Since such accumulator must not only be of large capacity but must also be capable of withstanding relatively high pressures, it constituted an expensive element, particularly for smaller size mills.

I have now found that practically the full advantages of such a system may be achieved utilizing an improved form of apparatus and liquor circuits, thus materially reducing installation and operation costs. It is particularly to be observed that while no element in the unit per se is new, such elements are specially correlated to produce a beneficial result in a novel mode of operation.

An object of the present invention therefore is to devise an improved method of digesting fibrous materials.

Another object is to insure marked economies in a hot acid system of pulp digestion.

A further object is to insure optimum utilization of steam not only as a heating but also as a power medium With these and other equally important and related objects in View, the invention comprehends the concept of thoroughly saturating fibrous material under optimum conditions of temperature, pressure and concentration with a simplified and efiective method and apparatus.

In order more clearly to describe the invention a typical physical embodiment is diagrammatically shown in the single accompanying drawing.

Briefly considered the invention comprises a treatment of fibrous materials which includes contacting the chips for a predetermined time with acid or equivalent digestion liquor at high temperature and pressure while recirculating the liquor to and through the material. The temperature of the entering acid is raised, and in some circumstances the liquor is circulated by means of direct steam. The temperature during the preliminary or soaking period is raised from 25 or 30 C. to approximately 115 C., i. e. to just below reaction temperature. In these circumstances, that is to say when chips have been contacted with recirculating hot acid liquor, under high pressure for a suflicient time, they are thoroughly permeated with hot acid and the temperature throughout the mass of chips is equalized. Upon subsequently raising the mass reaction temperatures uniform cooking As intimated above, installation and operating economies are attained by utilizing direct steam not only as a means of heating the acid but also, by utilizing an injector action, to force such acid liquor-from a storage tank to the digester. As will-appear more fully hereinafter this reduces the capacity requirements 'of the high pressure accumulator. The injector action of the steam is availed of up to its effective maximum and a pump is then cut into the feed line or recirculation circuit so as to insure cyclic flow of high temperature acid during the precirculating period.

In the present invention a steam injector is employed to heat the digester liquor and to force it through the acid feed and/ or recirculation lines only while certain conditions obtain. It has been found that it is substantially impossible to circulate cooking liquor with a steam injector when the temperature of the liquor is higher than 80 C. In the present invention therefore the steam injector is utilized for heating the entering stream of acid, forcing this acid to the feed, line into digester, and in some circumstances for recirculating the acid liquor to and from the digester. As pointed out above, improved results are secured if the temperature of the soaking liquor is raised to a relatively high degree while being maintained below reaction temperatures. In other words it is found that increased speed and uniformity of penetration is secured if the acid liquor is raised to a temperature of the order of from 100 to 115 C. In these circumstances a steam injector is not available for recirculation purposes. In accordance with this invention provision is made for cutting the pump into the feed line or the recirculation line when the temperature of the acid is increased above the amount stated for the purpose of either feeding additional quantities of hot acid into the digester or for recirculating such hot acid.

In these circumstances, therefore, a number of different types of operations may be carried out. In one type of operation relatively cool acid may be withdrawn from the ordinary low pressure storage tank and forced by means of a steam injector into the digester to fill the latter. During this forcing period the entering steam increases the heat of the acid. This operation may be so controlled that the digester is filled with acid directly from the storage tank. During such filling the temperature may be raised up to approximately 60 to 75". C. After the digester has been filled the storage tank may be cut out of the circuit, by operating a valve, and the liquor circulated directly to and from the digester by means of the steam injector. As noted above, this type of operation is available so long as the temperature of the acid is not increased substantially above 80 C. This type of feeding and precirculating using steam as the motive force, as well as the heating medium, may be continued for a relatively protracted period of time and thereafter circulation may be stopped and additional steam.

throughout with hot acid, and the temperature of the acid throughout the entire mass of chips is substantially uniform. 'I'hereupon additional heat units may be added to the mass within the digester to bring it up to reaction temperature, that is to say to cook the material. During the cooking period the acid may, if desired, be recirculated, utilizing the same recirculation circuit.

In another mode of operation the cold acid may be withdrawn from the storage tank and forced into the digester by means of .a steam injector, being preheated with steam in transit. Simultaneously acid may be pumped from the high pressure acid accumulator into the feeding line so as to more quickly charge the digester. In this operation the pump in the accumulator-feedingline supplements the action of the steam injector. Thereafter, that is to say when the digester has been filled with liquor at the desired pressure, the feed pump may be cut out and liquor recirculated directly to and from the digester under the action of the steam injector. This recirculation is continued up to the point when the acid liquor reaches a. temperature of substantially C., at which time the pump in the digester recirculation line is cut in.

It will be observed that with this type of operation installation and operating economies are achieved. The liquor required for filling the digester may be withdrawn directly from the storage tank and a considerable number of heat units imparted to it in transit to the digester. This materially diminishes the load requirements on the high pressure accumulator, permitting the use of a much smaller element for this purpose. The accumulatorin the. system is utilized as an acid storage vessel and as an acid preconditioning vessel. Hot relief gases and liquors withdrawn from the digester during soaking and/or digestion may be passed to the liquor within the acid accumulator, thus increasing the temperature and the free acid content of this liquor. This type of operation likewise diminishes the load on the feed pump to the extent to which the steam injector is used.

It is also particularly to be observed that this type of operation also presents other advantages. When the steam injector is used in a separate cycle or stage for filling and for precirculation, it will be observed that the soaking period then in effect comprises two stages. In the first relatively low temperature acid, i. e. at approximately 70 C. or below and of relatively low free acid chips for a given period. of time. Thereafter recirculation is continued under pump pressure acorns state is attained the forced or pump circulation of hotter acid is then brought into play.

During the operation, as will be more fully explained hereinafter, the energy of the steam flowing through the injector maybe utilized not only to force acid into the digester but it also may be employed to withdraw relief fluids from another digester undergoing cooking and admix these with the entering stream of acid, thus increasing the temperature and concentration of the entering acid.

A more complete understanding of the invention will be had from a consideration of the apparatus illustrated in the accompanying drawing.

The unit may comprise as major elements the cold acid storage tank I, the hot acid accumulator 2 and the digestors 3. While only one digester has been shown on the drawing, it will-be understood that this is indicative of a. battery of any desired number. When the digesting liquor employed is a metal bisulphite, this may be forced to the acid storage tank from the acid towers through the line 4 by means of the pump 5. Such acid is at relatively low temperature. If desired it may be given a mild degree of preheat by means of a closed heating coil near the bottom of the tank. The acid in the storage tank is preheated to some degree by means of direct contact with and absorption of hot low pressure gases, in the manner more fully to be explained. Since in most circumstances the acidin the storage tank is at atmospheric pressure, the degree of preheat that may be imparted to it is definitely limited for, as known by those skilled in the art, the temperature cannot be raised too high without evolution and loss of S02.

As indicated above, the accumulator 2, which is preferably spherical in shape, serves as chest regenerating plant in which heat units of the hot fluids discharged from the digester are imparted to a body of cooler liquor admitted from the stor. age tank. This cooler liquor absorbs the free gas evolved during the cooking and-"thus conserves chemical values.

The accumulator may be so constructed that the temperature, pressure and concentration con: ditions may be accurately controlled. The relatively cool acid from the storage tank is forced by means of pump 5 through line i and check valve i into eductor 8 and thence through the drop leg Q to the second eductor iii. From here it is discharged through the branch ii into'the acid within the accumulator. The eductor 8 is in communication with the high pressure header iii. During the operation of the plant, as will be understood, the cold acid flowing through line "i aspirates and absorbs the gases flowing through line it and thus imposes a degree of suction on the line it. Theveiocity of the cool acid flowing through the eductor 8 may be controlled by the valve i3. similarly, the flow of acid through the eductor it is controlled by valve it. The acid then flows down through the drop leg t and through the eductor it into the accumulator. This second eductor supplements the action of eductor ii in absorbing the free gas and in concomitantly preheating the cooler acid.

In order to insure maximum absorption of gas any desired velocity through the eductor ill and back into the accumulator through line H. During this passage the liquor intimately contacts with fluids flowing into the eductor through drop leg 9. To control the temperature of the liquor within the accumulator a'thermostatically controlled heater may be interposed in the accumul'ator recirculation line.

The accumulator is provided with gauge glass l5 and, if desired, with automatic liquid level indicating or recording devices as well as thermal and pressure indicating and/or recording units. The pressure within the accumulator may be automatically controlled within a relatively narrow range. This may be accomplished, for example, by means of the low pressure gas line i6 passing from the top of the accumulator into the liquid within the storage tank. This is provided with the pressure regulating valve Hi. This may be set at any predetermined value so that when the pressure within the accumulator rises above a certain point gases are automatically vented to the storage tank l. Similarly a bypass line H, having a pressure regulating valve ii, is provided between the high pressure header it and the accumulator. This valve may be set at any desired pressure difierential, say of the order of from 1 to 3 or 4 pounds. Thus when the pressure in the header it exceeds this pressure differential in the accumulator the valve ii operates automatically and fluids are bypassed around eductor i and sent directly to the accumulator.

Included in the unit is one or more digesters 3. These may be of the type generally employed in the art comprising a metal shell and a lining oi-sorne acid resisting material, such as suitable ceramics and the like. The digesters are each provided with a removable manhole cover 28 and blowoff connection iii. The cligester is provided with a low pressure overhead relief line 22 having interposed valve 22. This communicates with the low pressure header 23 through which low pressure gases flow to the acid storage tank to be absorbed therein. Connected in the low pressure relief line 22 (or at any other suitable point on the digester) is the air vent line 2 5 having control valve 26'. The digester is also provided with the overhead relief line 25, controlled by valve 25' which, as shown, communicates with the main header it. The side relief line 2%, having interposed control valve it, leads from a relatively low point of the digester to the main header it. It will be understood that the intake of the side relief line it is provided with the usual strainer means so as to prevent passage of fiber through the line. When the valve 26' is operated the liquid level within the digester may be lowered to any desired degree. To indicate this level the gauge glass 2? and/or a liquid level indicating or recording device maybe employed. The digester is also provided, in the manner well known to those skilled in the art, with temperature and pressure indicating and/or recording well known to those skilled in the art. Under the action of the steam flowing through the line 32 liquor is drawn throughthe acid line 30 and forced through the outlet line 33 and thence 5 through the branch 34, controlled by valve 34",

into the bottom of the digester and, preferably,

although not necessarily, into the blowoff connection.

Hotter acid liquor may also be forced into the digester from the acid accumulator. As shown,

the accumulator is provided with a drawoflf line 40, controlled by valve 40'. Interposed in the line 40 is a suitable pump 4|. The feed line 40 communicates with the acid line 30, as shown, so that liquor flowing through the line 40 enters the steam injector and is there subjected to the forcing effect of the steam jet.

In accordance with the principles already defined, means are provided to circulate acid liquor during the saturating period by means of the steam injector and/or a pump and, if desired, during the digestion or cooking period by means of the pump alone. As shown, the recirculating line 50, having an interposed valve 50', enters the digester at a high point thereof and in the portion within the digester is foraminated or otherwise formed so as to withdraw liquor free from fiber.

. Line 50 communicates with the pump 5|. Liqnor withdrawn through the line 50 andpump 5| is forced through the heat exchanger 52 and eductor 53 into the lower portion of the digester. The branch line 54, controlled by valve 54', communicates with the inlet side of the steam injector, as for example by being connected to 7 line 40.

Means are also provided to separately withdraw gases from the upper portion of the digester and re-introduce such gases into the recirculating stream of liquor entering the digester. As shown, the gas drawoff line 60, controlled by valve 60', communicates with an upper'portion of the digester and with the eductor 53. The velocity of the liquor passing through the eductor may be controlled by valve 53. It will be appreciated that when the pump recirculation cycle is in operation, that is to say when liquor is with drawn through line 50, as by means of the pump 5|, and forced into the digester, gases may be withdrawn from the upper portion of the digester and commingled with the recirculating stream so as to effect reabsorption of such gases.

As explained heretofore, the forcing effect of the'steam injector 3| is utilized only under predetermined temperature conditions. During operation when the temperature of the liquor is increased above a predetermined amount it is substantially impossible to recirculate the acid. Hence in order to utilize the full effect of the steam injector while nevertheless insuring substantially continuous recirculation, means are provided to automatically cut out the steam injector and cut in the recirculating pump when a predetermined temperature, say of the order of between 75 and 80 C., is reached. As shown, a thermostatic element I0 is positioned at some proper or convenient place within the digester. This thermostat is in circuit with the electrically controlled valve .ll, associated with the steam supply to the injector. The circuit is also connected, as shown, with the pump 5|. Thus when the temperature of the acid stream has reached a predetermined point at which the circuit is set to operate, the injector 3| is cut out and the motor 5| cut in.

As noted on the drawing, the'dig'est'er is pro vided with the steam inlet lines 80, controlled by valves 30'. These communicate with the main steam header and are utilized for the purpose of introducing steam to raise the mass to cooking temperature, in the manner well known.

As indicated hereinbefore, the steam injector 3| may be utilized to perform a triple function. For example, besides employing this to preheat the entering liquor and to circulate such liquor to and from the digester, the jet action of the steam may also be employed to aspirate or suck gases from one or more of the elements of the apparatus. For this purpose, as shown, the injector may be associated with a line. 36 which communicates with the low pressure or suction side of, the injector and may be selectively coupled at its other end either to the high or low pressure headers [B or 23 or directly to the top of a digester, operating either on the soaking or cooking period. In this manner, that is to say by connecting the injector to a source of gas, the acid concentration of the entering stream of liquor or the recirculating stream may be built up and concomitantly additional heat units may be imparted to the liquor.

It will now be appreciated that the several types of operation hereinbefore described may be advantageously carried out with this apparatus. In operation a particular digester 3 may be filled with chips by removing the manhole cover 20 and feeding in the chips under gravity or by means of a, ch p packing device in the manner known to those skilled in the art. After the chips have been admitted, in the desired manner, the manhole cover is bolted on. Valves 22', 2| and 60 are closed and valve 24' is opened. The valves in the digester recirculating line are closed and valves 34', 30 opened. Steam is then admitted through line 32. This steam flowing at high velocity into the injector aspirates or 1 induces a flow of liquor from storage tank I through line 30. This liquor is forced through lines 33 and 34. into the bottom of the digester. During this period the temperature of the acid liquor is raised due to the direct contact with steam. As the acid liquor enters the digester and flows upwardly therein the air content in the digester and in the intracellular spaces of the chips is displaced and vented through the open valve 24'.

When the digester has been filled to the desired level the valves 24' and 2| are closed, thus sealing the digester. Flow of steam through the line 3| may be continued, thus forcing in additional quantities of preheated liquor until any desired hydrostatic pressure is built up on the acid in the digester. When this point is, reached valve 30' is closed and valves 50' and 54' are opened, valve I'- being closed. In these circumstances continued introduction of steam through line 32 induces a flow of the line 32 is discontinued and the recirculating pump 5| is cut in. In these circumstances liquor preheated acid through the recirculating circuit is withdrawn from the top of the digester through line 51'! and forced thence through the eductor 53 into the bottom of the digester. At this time, or any time thereafter, the valve 66 5 may be opened. In these circumstances the gases in the vapor space of the digester are aspirated through line 60, due to the action of the eductor,

and are reabsorbed in the recirculating stream of liquor and reintroduced into the body of liquor in the digester. With this type of operation,

therefore, the acid concentration of the liquor in the recycle stream may be maintained at a high point.

Whenthe steam injector M is cut out and the pump 5i cut in so as to initiate the pump circulation, the temperature of the liquor is preferably raised. As has been pointed out, a desideratum in the present process is to completely saturate the chips with high temperature acid. In this 0 manner subsequent cooking is not only expedited but the resulting pulp is much more uniform.

, In accordance with the present invention, therefore, when the pump circulation is established heat units may be added by means of the heat 25 exchange 52, or if desired by introducing direct steam through the line 88. The heating medium employed in the heat exchange may be of any suitable kind, such as superheated steam, hot spent liquors and the like. Inthese circum- 30 stances, therefore, the chips are contacted with a recirculating stream of acid, the temperature of which is gradually increased from 70 or 80 C.

up to the desired soaking temperature, which may be as noted of the order of from 100 to Recirculation at this relatively high temperature is maintained until the chips are uniformly saturated or permeated with the hot acid. The time of this precirculation-soaking period may 40 vary, depending upon the character of the chips,

the speed of flow of the recycle stream and the like. In ordinary circumstances the total precirculating-penetrating period may vary from an hour to four hours or more.

45 When the chips are thoroughly saturated the conditions within the digester are adjusted to initiate cooking. In the preferred operation, particularly when a high hydrostatic pressure is carried during the soaking period, the liquid level 50 in the digester is dropped by withdrawing a certain quantity of liquor through the side relief line 26. Thereafter direct steam is introduced through the line 80 to quickly bring the mass up to reaction temperatures. It will be appreciated 55 that since the chips are thoroughly saturated with hot acid practically just below reaction temperatures, the quantity of steam required and the time necessary for initiating the cooking is quite short. Furthermore, due to the thorough pene- GO tration of the chips with the hot acid, the reaction is substantially uniform throughout each chip. This uniform reaction or cooking, as will be appreciated, minimizes and practically eliminates the shives or uncooked core portions of the 65 chips which have heretofore represented substantially direct losses.

The operation carried out during the cooking process will be understood. During the progress of the cook the liquid level tends to rise due to the 70 increased volume of condensate. This liquid level may be adjusted to the desired value by closing the top relief 22 and opening the side relief 2% sometime after reaction temperatures have been reached, usually at the fourth or sixth hour. By

75 the proper manipulation of the valves 22 and 26 Cal the desired level of liquid may be maintained in the digester.

As the reaction proceeds a considerable quantity of free gas is evolved due probably to the fact that the organic acids produced in the reaction 5 tend to displace and set free an equivalent amount of free sulphurous acid. This accumulation of gas would tend to build up an excess pressure. Under the present operation this pressure is relieved by opening valve 22 and recover- 1o ing the chemical and heat values by absorbing such gases in the liquor flowing through the eductor 8 and accumulator feed line 8. This relief is controlled, as known by those skilled in the art, so as to maintain the pressure substan- 15 tially constant down to the blow-down period. According to the present operation such pressure during the cooking may be maintained at about 15 pounds gauge and at an optimum temperature of about 135 C.

Steam is shut on sometime prior to completion of the cook in the manner well known. Thereafter the pressure is gradually diminshed down to avalue of about twenty or thirty pounds, after which the material may be discharged into a blow-pit through a blowofi line (notshown). If desired, before discharging the pulp from the digester it may be given a preliminary water wash within the digester and then discharged.

The cooking or digestion process preferably is carried out under conditions of a low liquid level and. minimum quantity of acid as more particularly explained in copending application of T. L. Dunbar Serial No. 758,314, filed December 19,

As explained, the present apparatus is susceptible of alternative modes of operation. Thus'in lieu of the operation described one might be carried out in which, during the initial filling period, acid from both the storage tank and the accum- 40 ulator are used. Thus in this method of operation to fill the digester valves 2|, 22', 60, 26', 50' and 54' are closed and valves 24', 30', 3d and '30 are opened. Steam is then admitted through line 32 and pump M is operated. In these circumstances cold acid from the acid storage tank and hot acid from the accumulator are commingled and forced, under the action of the steam jet, into the digester. The entering acid from the storage tank is thus preheated by direct contact with hotter acid from the accumulator and with steam. During this introduction the vent s valve 2d at the top of the accumulator is opened so that air displaced during the admission of the acid is discharged. When free gas begins to be 55 discharged at the air vent 25. this may be closed and valve 2 l opened so as to discharge such gases to the liquor in the storage tank. When the digester is filled or substantially filled valve M is closed and the introduction of acid by way of lines 38 and i0 is continued until the desired hydrostatic pressure is established. When this point is reached valves 30' and 40' may be closed and valves 50 and 54' opened, the introduction of steam through line 32 being continued. At this stage of operation steam entering the jet aspirates liquor through the lines 50, 5t and 46 so as to effect'recirculation with steam. This operation is continued, in' the manner described. until the temperature of the acid increases to the point where the motor and injector circuit operate. From this point the mass of liquor in the digester is directly recirculated under the actionof the pump 5i. During this recirculation the temperature of the acid may be increased by imparting heat units through the medium of the heat exchange 52 or by any other suitable method. Also during this period, when desired or necessary, valve 6| may be opened so that free gases forming in the upper part of the digester are separately recirculated and reabsorbed in the liquor.

If desired, in accordance with the principles of the invention, other methods of operation may be employed. Thus a two-stage soaking or penetration period may be carried out by first filling the digester with acid through the circuit 3IJ 3|--34 and then recirculating this acid directly to and from the digester until the temperature is raised to the point where the injector is no longer effective for recirculation purposes.

Thereafter, before pump 5| is operated to effect pump recirculation, the valve 40 may be opened and pump 4| operated to force in additional quantities of hot acid from the accumulator and thus build up the hydrostatic pressure of the acid to any desired degree. After this point is reached operation of pump 4| may be discontinued and pump 5| started so asto positively force the liquid to and from the digester. In

this type of operation therefore it will be appreciated that in the first stage the chips are contacted with a recirculating stream of acid at relatively low hydrostatic pressure and relatively low 0 temperature, that is to say of the order of from 70 to 80 C.

y In the second stage of the operation the chips are contacted with a recirculation stream of hotter acid at a higher hydrostatic pressure. In

is carried out in two phases, in the first phase utilizing steam as the forcing medium under relatively low temperature and low pressure and in the second stage at the higher temperature and higher pressure, and of course a correspondingly higher acid concentration since the higher pressure. maintained on the mass of acid enables the use of a greater quantity of free S02.

It is particularly to be observed also that the provision of the line 36 presents peculiar advantages in this type of operation. Since the entering acid is directly contacted with steam, some dilution of the acid obtains. This can to a considerable degree be compensated for by admitting acid gas, 1. e. S02, through the line 36 into the entering stream of acid.- This gas, being relief gas withdrawn from the digester or the accumu lator, or one of the gas lines, is in a hot condition. This introduction, therefore, not only tends to maintain the concentration of the acid but also to increase its temperature.-

After filling the digester and recirculating the acid in the several manners described, the material within the digester may be cooked in the manner given and preferably according to the method described in copending application of T. L. Dunbar Serial No. 758,314 mentioned above.

It will be understod that in the several treatments described maximum heat economies may be secured by the provision of suitably positioned heat exchanges; Thus, for example, the entering acid forced from the storage tank to the digester may be preheated in transit by means of the heat exchange 8| interposed in the acid line 30. Thus the acid liquor flowing through line may be increased in temperature to any desired degree. The heating medium utilized in the heat exchanger 8| may be steam, hot spent liquor and the like. Similarly acid entering the digester other words, the hot acid penetration treatment from accumulator 2 through the line 40 may be preheated by utilizing the heat exchanger 22.

In manycircumstances advantageous results are secured by utilizing a special two-stage treating process. In this type of operation soakingliquor is withdrawn from the acid storage tank and/or accumulator and forced into the digester by means of the steam injector 3| in the manner described. During the filling operation the valve 24' is opened so as to vent air and when the digester is substantially completely filled this valve is closed. When the digester has been filled with hot acid, under the desired hydrostatic pressure, this acid may be recirculated to and from the digester by utilizing-the steam injector 3|, in the manner described, and where a high temperature soaking operation is desired the pump 5| may be cut in after the temperature of the acid has increased to or above '75 to C. After the mass of chips is contacted with a hot recirculating stream of acid for a predetermined period of time this acid is completely withdrawn from the digester through the drawoff line. This type of process presents the advantages of leaching out the extractives and compacting the chips. In these circumstances the amount of liquor subsequently admitted and necessary for actual cooking is considerably diminished. Furthermore, the second batch of acid .may be admitted at a relatively high temperature and as close to cooking temperature as is feasible with a given installation, thus further reducing the period of time required to bring the mass up to reaction temperature.

In this type of operation after the preliminary soaking period the acid within the digester is Withdrawn through the line I33, controlled by valve 83'. This line is provided with the usual strainer 83" so as to serve as a filter and retain the fibrous mass within the digester. During this withdrawal the valves at the top of the digester are closed, that is to say valves 22, 2|, 60', 24', 26 and 50 are shut. In these circumstances withdrawal of the hot acid liquor establishes a partial vacuum within the digester. This existing vacuum greatly facilitates the subsequent admission of the hot cooking liquor, as will be appreciated.

The soaking liquor withdrawn from the digester may be reused in the process. Thus, for example, this liquor may be withdrawn through the branch 84, controlled by valve 84', and admitted to the line I! at the suction side of pump l3, thus being admixed with the recirculating stream of acid passing to and from the accumulator. Such acid, therefore, commingled with the acid in the accumulator, passes through line I4, eductor l0 and back into the accumulator. If desired, acid withdrawn from the digester through line 83 may be passed into the recirculating stream in line H. In these circumstances the valve 8! is closed and valves and 86' opened. Pump 85 is operated, thus drawing liquor through line 83 and forcing it through line 86 into line H.

The 'ac id then passes through the eductor, is-

contacted and commingled with the fluid passing down through drop leg 9 and is then discharged into the acid accumulator.

Such hot soaking liquor may also be utilized to preheat the acid within the storage tank. As shown on the drawing, this may be done by closing valves 84 and 86' and opening valves. 85' and 81'. When the pump, 85 is operated liquor withdrawn from the digester 3 through line 83 will be forced by way of pump 85 and branch 81 into the body of liquor within the acid storage tank. Preferably the line 81 terminates at a low point within the acid storage so as to insure the maximum absorption of gases. It will be appreciated that when the pump 85 is operated the liquor within the digester is discharged at a rapid rate in view of the supplemental action of the hydrostatic head of the acid in the digester and the force of pump 85.

It will be seen that this type of process presents many advantages. As indicated, it enables the use of a smaller accumulator while nevertheless achieving substantially the full advantages of a large acid accumulator system. Furthermore it cuts down the pump requirements for digester filling and recirculation purposes. The operations are so carried out that the steam injector' is utilized up to its point of optimum efiiciency.

While preferred apparatus and methods of operation have been described, it is to be understood that these are given merely to exemplify the underlying principles involved. Obviously modifications in structure and circuit flow will occur to those skilled in the art and such modifications are intended to be comprehended by the present disclosure. Hence the invention is not intended to be restricted to the apparatus or processes described, except as such limitations are clearly imposed by the appended claims.

I claim:

1. A process of digesting fibrous material which comprises forcing acid from a storage vessel into a digestion vessel by means of a steam injector, recirculating the liquor to and from the digestor with the injector and then recirculating the acid by means of a pump when the temperature of the acid has been raised to a predetermined degree.

2. In a process of digesting fibrous material that improvement which comprises contacting the material for a predetermined period of time with a body of digestion liquor maintained at relatively low temperature and under relatively low hydrostatic pressure, recirculating the liquor through the material by utilizing steam under pressure, and then contacting the material with a body of digestion liquor maintained at a higher temperature and under greater hydrostatic pressure and recirculating such high temperature liquor through the materialunder pump pressure.

3. In a process of digesting fibrous material in which hot acid liquor maintained below reaction temperature is recirculated through a mass of the material in a digester by means of a steam injector, that improvement which comprises utilizing the suction in the injector to withdraw hot gases from another vessel and commingle such gases with the recirculating digestion liquor, then recirculating the liquor to and from the digester by a pump pressure and absorbing hot gases in such pump circuit.

4.111 a process of the character described, charging a digester with fibrous material, forcing relatively cool acid from a storage tank into the digester while preheating the liquor by passing the liquor through a steam injector, then utilizing the steam injector to recirculate the body of liquor directly to and from the digester, and

utilizing the suction established in the injector to add hot acid gas to the recirculating stream, then discontinuing such injector recirculation when the temperature of the liquor has attained a predetermined value and forcibly recirculatin the liquor through a pump circuit and absorbing evolved gases in such circuit.

5. In a process of digesting fibrous materials, charging a digester with fibrous material, forcing relatively cold acid liquor from a storage tank into the digester while preheating the liquor by passing the liquor through a steam injector and simultaneously pumping hot preconditioned acid from a pressure vessel into the digester.

6. In a process of digesting fibrous material, charging a digester with the material, forcing relatively cold acid from a storage tank into the digester while preheating the liquor by passing the liquor through a steam injector, simultaneously withdrawing hot preconditioned liquor from a pressure accumulator and admixing such hot liquor with thecooler liquor from the storage tank, and discharging the mixed liquor into the digester.

7. A process of digesting fibrous material which comprises admitting the material to an enlarged chamber, introducing digester liquor at elevated but below reaction temperature to the digester; recirculating the liquor directly to and from the digester by means of a steam injector, then discontinuing the operation of the injector and recirculating the liquor to and from the digester by means of a pump, and then cooking the material.

8. A process of digesting fibrous material which comprises admitting the material to an enlarged chamber, introducing digestion liquor at elevated but below reaction temperature to the digester, recirculating the liquor to and from the digester while increasing the temperature thereof by utilizing a steam injector, then when the temperature of the liquor has reached a predetermined value automatically discontinuing the operation of the injector and continuing the circulation under pump pressure.

9. A process of digesting fibrous material which comprises admitting the material to an enlarged chamber, introducing digestion liquor at elevated but below reaction temperature to the digester, recirculating the liquor to and from the digester by utilizing a steam injector, then when the temperature of the liquor has reached a predetermined value automatically discontinuing the operation of the injector and continuing the circulation under pump pressure, and increasing the temperature of the liquor during such pump recirculation.

10. A process of digesting fibrous material in which the material is subjected to soaking with hot acid liquor in a digester prior to cooking, that improvement which comprises soaking the material for a predetermined time with acid liquor below C., utilizing the steam injector to recirculate such acid liquor to and from the digester and then soaking the material for an additional period of time with hotter acid liquor at higher.

culating mass of liquor below 80 C., utilizing.

the steam' injector to recirculate such acid liquor to and from thedigester and then contacting the material for an additional period of time with a recirculating mass of hotter acid liquor. while recirculating such hotter acid liquor to and from the digester under pump pressure.

12. A process of digesting fibrous material which comprises charging a digester with fibrous material, forcing acid liquor from an acid storage tank into the digester by means of a steam injector, forcing an. additional quantity of acid liquor into the digester under pump pressure, then recirculating the liquor to and from the digester by means of the said injector.

13. A, process of digesting fibrous material which comprises charging a digester with fibrous material, forcing acid liquor from a storage tank into the digester by means of a steam injector, forcing an additional quantity of liquor into the digester, under pump pressure, recirculating the liquor to and from the digester for a predetermined time by means of the injector, and then recirculating the, liquor to and from the digester for an additional period of time under pump pressure.

14. A process of digesting fibrous material which comprises charging a digester with fibrous material, forcing acid liquor from an acid storage tank into the digester by means of a steam injector, indirectly heating such liquor in transit from the tank to the injector, and then recirculating the liquor to andfrom the digester by means of the said injector.

15. A process of digesting fibrous material in which the material is submitted to a soaking action with hot digestion liquor below reaction temperature prior to cooking, that improvement which comprises circulating the liquor to and from the digester by means of a steam injector for a period of time suflicient to insure thorough penetration of the material with hot liquor, then withdrawing the liquor from the digester while retaining the digester closed so as to establish a partial vacuum within the digester, and subsequently admitting hot digester liquor to the digester and. cooking the material with such liquor.

16. A process of digesting fibrous material which comprises charging a digester with the material, forcing acid liquor from a storage tank into the digester by means of a steam injector, forcing an additional quantity of liquor into the digester under pump pressure, recirculating the liquor to and from the digester for a predetermined period of time by means of the injector, then recirculating the liquor to and from the digester for an additional period of time under pump pressure and then raising the mass to reaction temperature.

17. A process of digesting fibrous material which comprises charging a digester .with the material, forcing acid liquor from a storage tank into the digester by means of a steam injector, indirectly heating such liquor in transit from the tank to the injector, recirculating the liquor to and from the digester by means of the said injector until a predetermined temperature is reached and then continuing such recirculation under pump pressure.

18. In a process of digesting fibrous materials, charging a digester with the material forcing relatively cool acid liquor from the storage tank to the digester While preheating the liquor by passing it through a steam injector, simultaneously withdrawing hot preconditioned acid liquor from a pressure accumulator and admixing such hot liquor with the liquor from the storage tank, discharging the mixed liquor into the digester mingled stream to the digester, recirculating the liquor to and from the digester for a predeter- I mined time by means of the steam injector, and then continuing the recirculation to and from the digester under pump pressure.

20. In a process of digesting fibrous material, charging the digester with the material, forcing relatively cool acid from a low pressure storage tank to the digester while preheating the liquor by passing it through a steam injector, recirculating the acid to and from the digester by means of the steam injector until a predetermined temperature is attained; then forcing an additional quantity of hot preconditioned liquor from a high pressure accumulator into the digester to increase the hydrostatic pressure on the 1iqu0r,'and then recirculating such liquor to and from the digester under pump pressure.

21. An apparatus for treating fibrous materials which comprises a digester, a storage tank,

., a liquor feed line connecting the storage tank to the digester, a steam injector connected in said line for withdrawing liquor from the tank and forcing it into the digester, means for recirculating liquor to and from the digester, said means including a conduit connected to the digester at different heights thereof and a pump in said conduit, and means operable automatically to cut in the said pump and cut out the said injector.

22. An apparatus for digesting fibrous material comprising a digester, a storage tank, a fluid conduit connecting the storage tank to the digester, a steam injector in said conduit, means for recirculating liquor to and from the digester, said means including a conduit, a pump and the said'injector, and thermostatic means associated with the injector and pump and operable to cut the pump in and the injector out 01' the recirculation circuit upon the attainment of a predetermined temperature in the digester.

23. In a process of digesting fibrous material in which the material is subjected to soaking with hot acid liquor in a digester prior to cooking, that improvement which comprises soaking the material for a period of time with digestion liquor below C., utilizing a stream injector to recirculate such liquor to and from the digester; then soaking the material for an additional period of time with hotter digestion liquor maintained at a higher hydrostatic pressure, while recirculating such hotter liquor to and from the digester under pump pressure; then reducing the pressure on the liquor and increasing the temperature thereof to digest the mass.

24. In a process of digesting fibrous material in which the material is submitted to a soaking action with hot digestion liquor below reaction temperature prior to cooking, that improvement which comprises circulating liquor to and from the digester by means of a steam injector for a period of time sufliciently prolonged to insure penetration of the material with hot liquor, then withdrawing the liquor from the digester while retaining the digester/ closed so as to establish a partial vacuum within the digester, subsequent- 1y admitting hot digestion liquor to the digester and cooking the material with such liquor; passing the said withdrawn liquor to a storage vessel and there admixing it with the liquor subsequently to be admitted for cooking.

25. A process of digesting fibrous material which comprises charging the digester with fibrous material, forcing acid liquor from the acid storage tank into the digester by means of a steam injector, recirculating the liquor to and from the digester by means of the said injector, draining the liquor from the digester and admixing it with liquor in said acid storage tank and admitting to the digester hotter digestion liquor and raising the mass in the digester to reaction temperatures.

26. A process of digesting fibrous material which comprises charging the digester with fibrous material, forcing acid liquor from the storage tank into the digester by means of a steam injector, simultaneously admitting hot preconditioned digestion liquor from a pressure accumulator to the digester, recirculating the digestion liquor to and from the digester by means of a steam injector, discontinuing the recirculation by the steam injector and recirculating the digestion liquor under pump pressure to and from the digester; withdrawing said liquor from the digester and admixing the withdrawn liquor with the preconditioned liquor in said pressure accumulator, admitting additional quantities of hot digestion liquor to the digester and raising such liquor to reaction temperatures.

ALBERT D. MERRILL. 

